1. to convert into a fossil; to preserve (organic remains) in rock or mineral form
Usage: British spelling; American spelling is 'fossilize'; often used in passive voice
2. to become outdated or rigid; to stop developing or changing
Usage: figurative use

Examples

• The ancient insects were fossilised in amber millions of years ago.
• Shells and bones fossilise more readily than soft tissues.
• The company's outdated policies have fossilised over decades.
• His thinking had fossilised, making him resistant to new ideas.
• Organisms are fossilised when buried quickly in sediment.
• Traditional customs can fossilise if they are not adapted to modern times.